This invention relates to a span adjustment device for adjustment of span weight in an automatic weighing apparatus in which the weight of an object is measured as the output of a displacement sensor. More particularly, the invention relates to a span adjustment device with improved means for applying and removing a weight used for span adjustment.
In general, in an automatic weighing apparatus, articles to be weighed are supplied to a weighing hopper and the total weight including the weight of the hopper is sensed electrically. When the resulting electric signal which is a measure of the weight of the articles has reached a predetermined magnitude, the supply of new articles is stopped and the articles of the desired weight are taken out from the weighing hopper. This operating cycle is repeated for obtaining batches of the articles of the desired overall weight.
FIG. 1 is a top plan view showing such an automatic weighing apparatus, and FIG. 2 shows the same apparatus in side elevation. In this automatic weighing apparatus, a plurality of weighing hoppers 2 are mounted in a circular array above the circumference of the top opening of a collecting chute 1. Each weighing hopper 2 is operatively associated with a weighing mechanism 3 designed to weigh out articles contained in the weighing hopper 2. A distribution table 4 is supported on an electromagnetically operated vibrator 5 for supply troughs 6 and is set into helical reciprocating rotation. The supply troughs 6 are mounted radially on the outer periphery of the distribution table 4 and are vibrated by corresponding electromagnetic vibrators 7. Sensors 8 are used for sensing the quantity of articles left on the distribution table 4 for control purposes. A pool hopper 9 is provided between each weighing hopper 2 and its associated supply trough. Each weighing hopper 2 and the its associated pool hopper 9 are provided with a drive unit 12 and levers 10, 11 adapted for opening and closing the hoppers 2, 9.
The computerized combinatorial weighing apparatus thus constructed operates as follows.
The articles are supplied from the distribution table 4 through supply troughs 6 into pool hoppers 9 and thence into weighing hoppers 2. The articles thus received in the weighing hoppers 2 are weighed by their associated weighing mechanisms 3. Based on the weights measured by the weighing mechanisms 3, a control unit, not shown, of the weighing apparatus performs a combinatorial weighing operation by comparing the result of each combinatorial adding operation performed on the article weights with a preset target weight and selecting the combination of articles, known as the best combination, that gives a total weight equal to the target weight or closest to the target weight. In this case, the number of articles in the combination may be arbitrary or predetermined, as desired. The control unit then operates to open the thus selected weighing hoppers 2 by the operation of the levers 10 so that the articles giving said best combination are released into the collecting chute 1 from the weighing hoppers 2 to be discharged towards a packaging machine or a bucket conveyor, not shown. This will leave the selected weighing hoppers 2 empty. Articles are then newly delivered from the corresponding pool hoppers 9 into said weighing hoppers 2, leaving these pool hoppers 9 empty, whereupon a new supply of articles is delivered from the distribution table 4 and the corresponding supply troughs 6. The weighing operation can be continued in this manner by repeating the foregoing steps.
With continued and prolonged use of the automatic weighing apparatus, however, the weights measured by the apparatus tend to deviate from the true value due to an aberration in so-called span weight (described below) corresponding to the measured weights, or in the zero-point indication. The aberration in in zero point or span weight is caused in turn by some of the articles affixing themselves permanently to the inside of the weighing hoppers, by changes in the weighing mechanisms themselves with time, or by changes in the ambient temperature. The result is a weighing error and diminished weighing accuracy.
The span weight referred to above may be understood from the following discussion. To perform a zero-point adjustment, the weighing hopper of a weighing mechanism is emptied and the indication given by the weighing mechanism is set to zero. Thereafter, the maximum load capable of being weighed by the weighing mechanism is applied. Let this maximum load be 1000 grams. When the load is applied, let us assume that the weighing mechanism indicates a weight of 910 grams. If the weighing mechanism indicates a weight of A grams during an actual weighing operation, therefore, then the true weight A.sub.t of the load will be found from the formula: EQU A.sub.t =(A/910).times.1000 grams.
The result of performing the above operation is the span weight.
To eliminate the problem of weighing errors, it is necessary to make frequent periodic adjustment of the span weight and zero point of the weighing apparatus. The span weight adjustment, described briefly above, will now be described in greater detail.
To effect a span adjustment, the weighing hopper of the weighing mechanism is emptied and the gain of the weight sensing circuit is adjusted by regulating a zero-point adjustment dial. Next, with the weighing hopper still empty and with a weight load, corresponding to the span weight, placed on the weighing mechanism, the gain of the weight sensing circuit is adjusted by regulating a span dial. When the span adjustment is made by a manual operation, the above weight load, which will be designated a reference weight for convenience, must be placed on and removed from the weighing mechanism for each adjustment operation. In consideration of the fact that much time and labor may be required for span adjustment especially with an automatic weighing apparatus having a multiplicity of weighing mechanisms, the pesent inventor has filed an application for an automated span adjustment device. In this device, a weight pan is secured directly or indirectly to a weighing hopper and the reference weight is hung above this pan by a pulley and a connecting member such as a string. The other end of the string is connected to the movable core of an electromagnetic solenoid which is supplied with current only during span adjustment to place the reference weight on the weight pan, whereby the reference weight is loaded on the weighing hopper.
In this prior-art device, however, part of the weight of the connecting member is loaded on the pan in addition to the reference weight, because the connecting member such as the string is connected to the reference weight. This may give rise to an inaccuracy in the span adjustment. When a slender connecting member of low weight is used, the connecting member stretches with prolonged use and may contact the pan even when the current is not actually supplied to the solenoid. The connecting member may also break.